In the manufacture of silicon single crystal by the Czochralski method (CZ method), a vitreous silica crucible is used. In the CZ method, silicon material is put into the vitreous silica crucible, and thereafter heated and fused. Thereafter, a seed crystal is dipped into the silicon melt, and the seed crystal is gradually pulled while rotating the crucible to grow a single crystal. In order to manufacture high-quality silicon single crystal of for semiconductor devices at low cost, it is necessary to improve the single crystal yield in one pulling process; therefore, a crucible with a stable shape without deformation during a long period of operation becomes necessary.
In deformation of a crucible, the straight body portion of the crucible falling toward the side of silicon melt, which is called inward sagging, becomes a particular problem. Although a heat shield plate which is called a hot zone is provided around the ingot near the liquid surface of the silicon melt, in a case that the straight body portion of the crucible inwardly sags, the inwardly sagging part may contact the heat shield plate. Since the crucible is rotating during the pulling of single crystal, the straight body portion of the crucible comes in contact with the heat shield plate while rotating; thus defects, such as a further deformation of the crucible, damage to the heat shield plate, and reduction of manufacture yield due to a mixing of crucible fragments in the silicon melt, occur.
A method of increasing Al concentration in an outer layer of a crucible to a high viscosity in order to prevent the deformation of the crucible at high temperature (refer to Patent Literature 1), is known. In addition, a method of outwardly widening a straight body portion of a crucible to prevent inward sagging (refer to Patent Literature 2) is also known.